AMI. ctwm. 1992, 6 4 , 528-533
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Use of Electrochemical Microscopy To Examine Counterion Ejection from Nafion Coatings on Electrodes Chongmok Lee and Fred C.Anson* Arthur Amos Noyes Laboratories, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
prepared by diluting a 5 wt % solution (Aldrich)with 2-propanol. Other reagent-grade chemicals were used as received. A glassy carbon disk electrode (Tokai Carbon Co.) with an area of 0.074 cm2was used as the Nafion-coated substrate electrode. It was sealed in heat-shrinkable tubing, force-fit to a Teflon holder, polished with 0.5-pm alumina, and sonicated in pure water before coating with Nafion. A carbon microtip electrode having a diameter of ll pm was prepared, mounted in glass, and polished as described in ref 4. Apparatus and Procedures. The scanning electrochemical microscope was of a design similar to that given in ref 3b. Ita performance has been previously described! Nafion coatings were applied to the substrate electrode by transferring 0.9-pL aliquota of the 0.5 w t % stock solution to the electrode surface with a microsyringe and allowing'the solvent to evaporate at room temperature. It was not possible to assure that all of the Nafion solution transferred to the small substrate electrode remained on the glassy carbon during the evaporation of the solvent. The quantity of Nafion present in the coatings was determined from the quantities of O~(bpy),~+ they incorporated when saturated with the complex. Voltammograms were recorded at a low scan rate (11mV s-') with coatings that had been fully loaded with O~(bpy),~+ and then scanned over the Os(bpy)2+/2+wave a few times to produce the constant corresponding to one Os(bpy)t+/z+ In a recent study of the rate of electron hopping between cation for every three SO3- groups. The area under the cathodic wave was measured, and the coating thickness was calculated by O~(bpy),~+ and O~(bpy),~+ (bpy = 2,2'-bipyridine) cations taking the density of Nafion as 1.3.6 The estimated thickness of incorporated in polyanionic coatings of Nafion on glassy the coatings obtained in this way was 0.2 f 0.05 pm. w b o n electrodes, a model was proposed which was reasonably Incorporation of Os(bp~),~+ into the resulting f i i was carried successful in accounting for the observed strong dependence out by immersing the coated electrode in a 1 mM solution of of the hopping rates on the concentration of the Os(bpy)33+/2+ Os(bpy)gP+for times sufficient to obtain the desired extent of couple present in the coatings.' Measurements were restricted ion-exchange (several minutes to many hours). f i r loading the to cases in which the total quantities of Os(bpy),2+ incorpoNafon coatings with Os(bpy)?+ the electrodes were washed with rated in the coatings were less than one-third of the total water and transferred to a 0.2 M sodium acetate buffer (pH = quantity of fixed anionic sulfonate sites within the coatings 4.6) to carry out electrochemical measurements. The quantities to ensure that complete oxidation of the O~(bpy),~+ cations of O~(bpy),~+ incorporated in the coatings were determined by coulometric assay from the areas encompassed by cyclic voltamto Os(bpy),3+ did not require expulsion of a portion of the mograms recorded at low scan rates or from measurementa of Os(bpy),3+ in order to maintain ionic electroneutrality within chronopotentiometric transition times obtained with low constant the highly cation permseledive Ndion coatings. The present currents. Potentials were measured with respect to a saturated study was designed to examine the extreme case where such calomel reference electrode (SCE). permseledive Nafion coatings were saturated with Os(bpy)gP+ The microtip electrode of the scanning electrochemical miso that a portion of the incorporated electroactive cations croscope was positioned very close to the surface of the Nafionwould have to be ejected from the coating during the oxidation coated substrate electrode to monitor the concentrations of of O~(bpy),~+ to Os(bpy)p in order to maintain equality electroactive ions released from the coatings. The positioning between the total incorporated cationic charge and the conof the microtip electrode relative to the substrate was carried out solution. The potential of the tip was with a 1 mM O~(bpy),~+ stant anionic charge of the fixed sulfonate groups within the maintained at 0.9 V where the diffusion-limited oxidation of Nafion. The ejection of electroactive reactants from the O~(bpy),~+ proceeded. The distance between the tip and substrate coatings was monitored with a microtip electrode positioned electrode was controlled via the micropositioning devices of the just above the coatings in an experimental arrangement similar scanning electrochemical micro~cope.~~J This distance was deto one introduced by Engstrom and co-workers.2 The posicreased until the current at the tip electrode began to increase tioning of the tip electrode with respect to the substrate because of the positive electrochemical feedback resulting from electrode was accomplished with a scanning electrochemical re-reduction at the coated substrate electrode (which was mainmicroscope of the type described by Bard and c o - ~ o r k e r s . ~ tained at a potential of 0.3 V) of the O~(bpy),~+ generated at the The results revealed several unusual features in the electrotip. When the feedback produced a doubling of the tip current chemical behavior of the saturated coatings which are dethe distance between the tip and substrate electrode was taken to be 0.62a (a is the radius of the tip electrode) on the basis of scribed in this report. the digital simulation and experimental data reported by Kwak EXPERIMENTAL SECTION and Bard? Other tip-substrate seeparation distancea were obtained Materials. Oe(bpy),Cl2.6H20 was prepared as previously from the calibrated Z-axis controller of the micropositioning described.' A 0.5 wt % solution of Nafion (eq wt = 1100) was device. Although the experimenta in ref 8 were obtained with The Ideal catlon permselectlvlty exhlblted by electrode coatlngs prepared from the polyelectrolyte Naflon leads to ejection of countercatlons from the coatings during electrochemkal oxldatlon of Os(bpy)t+ counterlons Incorporated In the coatlngs. If the coatings are saturated wlth Os(bpy):+ 80 that these are the only counterlons present, one-thlrd of the Incorporated catlons are ejected during the oxldatlon of Os(bpy):+ to Os(bpy)?+. The cyclic voltammetry of the Incorporated O~(bpy),~+'*+couple is altered substantlally In the absence of addltlonal counterlons. The changes are attributed to contrlbutlons to the voltammetrlc potentials from the free energy of transfer of these strongly bound counterh.Mkrotip electrodes posnloned just above Natlon-coated electrodes were used to monltor the ejection of both 06 (bpy)," and Os(bpy)t+ from Naflon coatlngs. Much more of the former complex lo ejected from saturated coatlngs whlch Is bolkved to be the result of electric fleld-adstod ejectlon.
0003-2700/92/0364-0528$03.00/0
0 1992 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992
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Nafkm coatlng on the glassy carbon substrate electrode. The extent of loading of the coating, Xa(=I'a/I'ms-) were 0.31 (curve I), 0.42 (curve 2), and 0.47 (cwve 3). The length of time the Nafbn coatings were exposed to a 1 mM solution of Oa(bpy);+ to obtain the quoted vekres of X, were 1 mln (cuve 1),58 min ( m e 2), and 11.5 h (curve 3). Supportkrgelectrolyte 0.2 M Cl-bCOONaadjusted to pH 4.6. Scan rate = 11.6 mV s-'. The initial potential for ail curves was 0.2 V.
uncoated electrodes, it was demonstrated in a previous studfb that the same analysis can be applied to coated electrodes 80 long as experimental conditions are chosen to aasure that the feedback currents do not exceed the maximum electron propagation currents that can be sustained by the coated substrate electrode. Such conditions were employed in the present experiments. After calibrating the distance scale of the Z-axis controller in the way just described, the tip electrode was retracted about 1 mm from the substrate and the solution in the electrochemical cell was removed by careful aspiration. The cell was washed several times with water, pure supporting electrolyte was introduced, and the tip was repositioned to carry out experiments in solutions free of Os(bpy)$+. Tests in which the original solution of Os(bpy)32+instead of pure supporting electrolyte was used to fill the cell after the washing cycle showed that the tip-substrate separation could be reproduced to within *1 pm.
RESULTS AND DISCUSSION Cyclic Voltammetry of Os(bpy),2+in Highly Loaded Nafion Coatings. Most previous studies have reported that the O ~ ( b p y ) ~ ~couple + / ~ + in Nafion coatings exhibits cyclic voltammetric r e a p o m which are symmetricaland correspond fairly closely to those expeded for a simple, Nernstian redox couple confined to the electrode s~rface.'.~However, very different behavior is obtained if the quantity of Os(bpy),*+ incorporated in the Ndion coatings exceeds one-third of the total anionic sites contained in the coating so that not all of the initially incorporated complex can be oxidized to Os(bpy)$+ within the Ndion without violating ionic electroneutrality within the coating (assuming that the Ndion coatings retain their ideal cation permselectivitylO). For example, in Figure 1is shown a seriea of cyclic voltammograms for a Nafion coating in which increasing quantities of Os(bpy)$+ were incorporated. The symmetrical current peaks near 0.62 V (Figure 1, curve 1) resemble those reported in previous studie~.'.~Such responses are obtained when the loading, X , = rhw/rm-(r,represents the moles cm-2 of the indicated ionic species in the coating), does not exceed 0.33. (Thiscondition was maintained during the recording of the cyclic voltammograms in the recent study of the Os-
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( b p ~ ) ~ ~ + / ~ + - N asystem f i o n from this laboratory.') However, as Xo, is increased above 0.33, the current-peak a t 0.62 V becomes smaller as a new peak near 0.88 V develops (Figure 1,curve 2), and when X, approaches its maximum value of 0.5, only the peak n w 0.88 V remains (Figure 1,curve 3). The anodic peak at 0.88 V disappears if the electrode is maintained a t 1.0 V for a few minutes before the voltammogram is recorded. A response resembling curve 1in Figure 1results and remains unaltered as the potential is cycled repetitively between 0.2 and 1.0 V. The origin of the new peak which develops near 0.88 V as XOsis increased above 0.33 becomes clearer if it is recognized that the electrochemical half-reaction which is responsible for the flow of anodic current when XO,5 0.33 must differ from the half-reaction when X b = 0.5. Thus, for XoSI0.33 the half-reaction is O ~ ( b p y ) ~ + ( S 0+~ Na+(S03-) -)~ - e- = O S ( ~ P Y ) ~ ~ + ( S+ONa+, ~ - ) ~(1) where represent sulfonate groups within the Nafion coating that are associated with each incorporated cation. For Xo, I0.33 a t least one-third of the sulfonate groups are associated with the other counterion present (assumed to be Na+ in this example). A portion ( X , < 0.33) or all (XO, = 0.33)of these much more mobile counterions are easily ejected from ideally permselective coatings into the supporting electrolyte (Na+,) during the oxidation of O ~ ( b p y ) ~ This ~+. reversible transfer of counterions between the coating and supporting electrolyte is expected to cause the apparent formal potential of the Os(bpy)$+/2+couple in Ndion to shift by 59 mV when the concentration of counterions in the supporting electrolyte is changed by 1order of magnitude. Such shifts were demonstrated in an earlier study.loa The relevant half-reaction when X , = 0.5 is O ~ ( b p y ) ? + ( S 0 ~-- )ne~ = 2/30s(bpy)33+(S03-)3+ (1- n ) [ O s ( b ~ ~ ) 3 ~ +1(n s - ~ / J [ O S ( ~ P Y ) (2) ~~+I~
In this case, O s ( b ~ y ) ~Os(bpy)$+, ~+, or both must be ejected from the coating in order for current to flow, and the fraction of the originally incorporated Os(bpy)?+ which is oxidized will vary between 67% (n = 2/3) and 100% (n = 11,depending on the ratio of Os(bpy)a+ to Os(bpy)l+ in the ejected cations. The important point to note is that the positive free energy of transfer of the ejected cations from the Ndion coating to the supporting electrolyte has to be supplied in order for the half-reaction to proceed. The great affiity of Nafion for both Os(bpy)p and Os(bpy)$+" means that the needed free energy of transfer will be substantial and this factor is reflected in the more positive potential required to accomplish the oxidation of the incorporated Os(bpy)$+ when X , 0.5 (Figure 1,curve 3). Other factors, such as the significant effecta from the electric fields present in the coatings,' would also be expected to contribute to the shift in potential, and the voltammetry shows the oxidation and reduction to be far from reversible (Figure 1,curve 3) so that no attempt was made to calculate the magnitudes of the expeded shifts in potential when X , 0.5. Nevertheless, the behavior shown in Figure 1 seems entirely consistent with expectations based on the difference between half-reactions 1 and 2. Ejection of O ~ ( b p y ) ~ ~from + / ~ Nafion + Coatings. To monitor the ejection of Os(bpy)$+ or Os(bpy)32+from Nafion coatings during the oxidation of the incorporated Os(b~y)3~+ counterions, the microtip electrode was positioned just above the coated electrode. The potential of the tip electrode was maintained either at 0.9 V, where any Os(bpy)?+ ejeded would produce an anodic tip current, or a t 0.3 V, where any OS(bpy)l+ ejected would produce a cathodic tip current. In separate experiments with an uncoated substrate electrode
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Flguro 2. Tip currents measured durlng cyclic voltammetry of Os(bpy)?+'*+ incorporated within a Nafion coating on the substrate electrode. The potential of the tip electrode was maintained at 0.3 or 0.9 V to monitor the concentratlon of Os(bpy)Z+ (I I I) or Os(b~y)~*+ (II), respectively. The tip was positioned ca.3.5 pm above the substrate electrode. Cathodic tip curents are plotted upward and anodic, downward. (A) The substrate electrode was loaded with Os(bpy),*+ and cycled several tlmes to decrease X , to 0.33. (B) First voltam mogem folkwing exposwe of the coeted substrate electrode to 1 mM Os(bpy):+ for 25 h ( X h = 0.5). Other conditions as in Figure 1.
it was established that, with the tip-substrate separation distances employed, species generated at the surface of the substrate electrode required less than 0.2 s to diffuse to the tip electrode where they were detected. In Figure 2 are shown the responses observed at the tip electrode during the recording of cyclic voltammograms at the coated substrate electrode. When X b I0.33, essentially no tip current was observed with the tip potential set to detect Os(bpy)$+ or O s ( b p ~ ) (Figure ~ ~ + 2A)which demonstrated that the cations ejected from the Ndion coating during the scan to more positve potentials were electroinactive sodium or hydrogen ions. However, when X, was increased to near 0.5, the anodic peak potential shifted to the more positive value seen in Figure 1 and substantial cathodic tip currents were observed during the oxidation of the O ~ ( b p y ) within ~ ~ + the coating (Figure 2B). The cathodic tip currents with the tip potential set at 0.3 V (curve 111) were much larger than the anodic currents with the tip potential at 0.9 V (curve II), which demonstrated that the cations arriving at the tip consisted primarily of O~(bpy),~+. The concentration gradient generated within the Ndion coating during the recording of the voltammogram in Figure 2B is expected to be quite small with the low scan rate (11 mV 8-l) and thin coating (-0.2 wm) employed because the diffusion coefficient of O~(bpy),~+ within Ndion coatings increases to unusually high values (10-8-10-7cm2 s-') when X, is high.' Thus,the ratio of the concentrationof Os(bpy)?+ to Os(bpy)$+ at the electrode/coating interface and at the coating/solution interface should remain essentially the same ae the voltammogram is recorded. The preferential ejection of O ~ ( b p y ) at ~ ~the + coating/solution interface, even at potentials where larger concentrations of O ~ ( b p y ) are ~ ~ present + at the interface (Figure 2B),suggests that the electric fields
present in the coating control the ejection process. The equilibrium affinities of Os(bpy)$+ and Os(bpy)?+ for Nafion do not differ substantially," so the preferential ejection of the more highly charged cation is most easily understood in terms of a field-assisted process. It was shown previously that the electric field contributes substantially to the large increase in the diffusion coefficient of the O ~ ( b p y ) , ~ +couple / ~ + in Ndion at high loadings.' The fact that the total faradaic charge required to oxidize a coating with Xa = 0.5 is ca.1.5 times greater than the charge required to oxidize the same coating after it has been cycled a few times between 0.2 and 1.0 V to produce a coating with Xa = 0.33' is consistent with minimal ejection of O s ( b p ~ ) ~ ~ + during its oxidation within Ndion coatings. If a significant portion of the cations ejected from the coating during the first oxidation cycle were O~(bpy),~+, the ratio of the faradaic charge consumed in the first and steady-state oxidation cycles would be smaller than the observed value near 1.5. Constant-Current Oxidation of O ~ ( b p y ) , ~in+ Highly Loaded Films. Additional insight into the behavior of the coatings was obtained from experiments in which constant anodic currents were passed through the coated substrate electrode while the microtip electrode was used to monitor species ejected from the coating. An advantage of this experiment is that it provides a clear indication of any delay between the initiation of the electrolysis at the coated substrate electrode and the ejection of Os(bpy)32+and Os(bpy)$+ from the coating. In Figure 3A are shown three, successive potential-time curves (chronopotentiograms) that resulted when a constant anodic current was passed through the substrate electrode with a Nafion coating loaded with as much Os(bpy),2+as it would accept, X b 0.5. The cathodic currents measured at the nearby tip electrode (with ita potential set at 0.3 V) are shown in Figure 3B (in Figure 3C the current sensitivity is expanded 10-fold to allow the time at which the tip currents begin to flow to be more clearly identified). During the recording of the first chronopotentiogram (l9) when the coating contained the most O~(bpy),~+, the anodic substrate current flowed for about 2 s before the cathodic tip current began to flow (Figure 3C). This delay is much longer than the transit time required for O ~ ( b p y ) released ~ ~ + from the substrate electrode to diffuse to the tip electrode which was measured as ca. 0.2 s in a separate experiment with an uncoated substrate electrode. The delay in the response at the tip electrode could reflect the initial preferential ejection of residual electroinactive counterions which remained in the coating or were introduced into the outermost portion of the coating by reverse ion-exchange with the cations in the solution of pure supporting electrolyte. In the latter case, these electroinactive counterions could carry most of the ionic current for a short period at the beginning of the electrolysis. In principle, the ionic current at the coating/solution interface could be carried by the ejection of O ~ ( b p y )instead ~~+ of O~(bpy),~+ counterions, but the very small tip currents observed with the tip potential set to detect Os(bpy)?+ during the cyclic voltammetric experiments of Figure 2 showed that ejection of O~(bpy),~+ is not an important factor in the maintenance of electroneutrality within the coatings. The eecond chronopotentiogramshown in Figure 3A ( m e 2.4 has a shorter transition time as expected because of the ejection of part of the incorporated complex during the first experiment. The corresponding tip current response (curve 2T) is delayed for a much longer period which is consistent with the condition: 0.33 < Xb < 0.5 that would result if only a portion of the Os(bp~)33+/~+ complex ejected during the f i t experiment were reincorporated when the substrate electrode potential was returned to 0.3 V for 60 s before the second chronopotentiogram was recorded. The third chronopoten-
ANALYTICAL CHEMISTRY, VOL. 84,NO. 5, MARCH 1, lQ92 531
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T I E [SCI Flgu. 9. Cathodic cwents recorded at a tlp electrode poskned 7.5 pm above the Nafion coating on the substrate electrode during constantcurrent oxidation of Os(bpy):+ incorporated in the Naflon. (A) Chronopotentiograms at the substrate electrode through which a constant anodic current of 3.75 p A was passed. The coating was e x p o d t o a l mMOs@pyh*sokrtknfor10.5hbeforetheRrstcvve (1,) was recorded. Curve 28 was recorded after the electrode used to obtain cuve 1, was cycled once between 0.2 and 1.0 V and then maintained at 0.3 V for 60 8. Curve 3, was recorded after the electrode used to record m e 2, was cycled once between 0.2 and 1 V, held at 1.0 V for 10 min, and then maintalned at 0.3 V for 80 8. (B) Cathodic tip currents measured while the chronopotentbgrams in A were recorded. The tip potential was maintained at 0.3 V. (C) R e display of Bwltha more senslthre ament scale. Supportine dectdyb: 0.2 M CH,COONa adjusted to pH 4.8.
tiometric curve was recorded after the substrate electrode had been held at 1.0 V for 10 min to decrease Xb to 0.33. The transition time was about two-thirds of the transition time for curve ls, as expected if X, had decreased from 0.6 to 0.33. Under these conditions no ejection of OS(bpy)?+ was detected at the tip eledrode; the ionic flux crossing the coating/solution interface was made up entirely of eledroinactive counterions. The magnitude of the constant currenta passed through the coated substrate electrodes affecta the delay in the tip current response as shown in Figure 4. The greater the substrate current, the shorter the delay time. The product of the substrate current and the delay in tip current response is approximately constant as would be expected if the delay refelected the time required for electroinactive counterions near the coating/solution interface to be removed from the coating under the influence of the constant anodic substrate currents. The abrupt increase in cathodic tip current that occurs shortly before the tmnsition time for the fully loaded substrate electrode (curve 1T in Figure 3B) reveals a large increase in the rate of ejection of O ~ ( b p y ) ~One ~ + . might have anticipated
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some change in the ratio of O ~ ( b p y ) to ~ ~O+~ ( b p y ) in ~ ~the + ejected cations as their ratio in the coating changed, but the sudden surge in Os(bpy)$+ arriving at the tip was surprising. A similar phenomenon can be seen during the cyclic voltammetry in Figure 1: The cathodic tip current surges as the anodic substrate current nears its peak value where most of the complex remaining in the coating has been converted to
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The behavior in Figure 3B is not the result of a sudden shift from the ejection of a mixture of O s ( b p ~ ) and ~ ~ +O~(bpy),~+ to pure Os(bpy)?+. As shown in Figure 5, the ejection of O a ( b p ~ ) is ~ ~much + less extensive than that of O s ( b ~ y ) ~ The ? ejection of O~(bpy),~+ produces considerably smaller anodic tip currents (Figure 5B),the ejection begins to decline after about 8 s, and it essentially ceases well before the surge in
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Chronopotentiogramswith current-reversal at the Nafloncoated substrate electrode and the corresponding tlp currents. (A) Chronopotentiogramsrecorded sequentially with an lnitlaily fully loaded coatlng, X , = 0.5. An anodic current of 3.75 PA was passed for 15 s followed by a cathodic current of the same magnitude. Conditions between each chronopotentlogram as In Figure 3. (B) Cathodic tip currents measured durlng the recording of the chronopotentlograms in A. The tip potentlal was maintained at 0.3 V and the tip was posltioned ca. 11 pm above the substrate electrode. Supporting electrolyte: 0.2 M CH,COONa adjusted to pH 4.6. Flgure 6.
O~(bpy),~+ occurs in Figure 3B. We speculate that this cathodic current surge is the result of the complete removal from the coating of all residual, mobile, electroinactive countercations which forces more of the ionic current flux to be carried by the only remaining countercations present in the coating, O~(bpy),~+. The current spikes in the initial tip current responses shown in Figure 5 are artifacts which result from greater electronic coupling between the tip and substrate circuits when the tip is set to detect O~(bpy),~+. The delay in the tip current response in Figure 5 appears only slightly shorter than that in Figure 3 which show that the former delay is not the result of the initial ejection of one oxidation state of the complex followed by the other. Thus, the origin of the delays in tip current responses for Figures 3 and 5 seems best ascribed to the initial ejection of electroinactive counterions. Current-Reversal Experiments. In Figure 6 are shown the results of reversing the direction of the constant current flowing though the coated substrate electrode for different values of Xa. For XOs 0.5 the Os(bpy),,+ ejected from the coating during the flow of anodic current diffuses away from the interface (curve lT in Figure 6B) and is only partially re-reduced when the direction of the current is reversed. As a result, the transition time following current reversal is shorter than the period of anodic electrolysis (Figure 6A, curve Is). A repetition of the experiment with the same coating (for which 0.33 < Xos < 0.5) yields an extended transition time (curve 2s in Figure 6A) because less Os(bpy)$+ is ejeded from the coating (compare curves lT and 2T in Figure 6B). Repeating the experiment a third time produced essentially no ejection of Os(bpy)$+ (curve in Figure 6B) because X, had been decreased to 0.33 and the reverse transition time became almost equal to the forward electrolysis time (curve 3s in Figure 6A). The change in tip current produced by the reversal of the substrate current with the coating for which Xoa 0.5 (curve lT in Figure 6B) is much more gradual than when the same experiment is repeated with an uncoated substrate electrode in a solution of O ~ ( b p y ) (Figure ~ ~ + 7B). The reason is that the O s ( b ~ y ) , ~ejected + into the solution sampled by the tip
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electrode can be consumed following the current reversal only by reincorporation into the coating as a counterion, an inefficient process in the presence of the much higher concentration of supporting electrolyte cations (Na+),or by reacting with O ~ ( b p y ) cations ~ ~ + in the coatings. Neither pathway proceeds with the 100% efficiency at which the reversed current consumes the Os(bpy),3+ present in the solution next to an uncoated electrode (Figure 7A).
CONCLUSIONS When all of the countercations in Nafion coatings consist of O s ( b p ~ ) complexes, ~~+ the oxidation of the O s ( b ~ y ) , ~is+ shifted to significantly more positive potentials than those where O~(bpy),~+ is reduced. The oxidation of Os(bpy),2+is accompanied by the ejection of Os(bpy)$+ cations, and many fewer O~(bpy),~+ cations, from the coating. When coatings contain no more than one O s ( b ~ y ) cation ~ ~ + for every three SO3- groups in the coating, the oxidation and reduction of the incorporated counterions ocm at the same potential and there is no significant ejection of either O ~ ( b p y ) or ~ ~Os(bpy),2+ + from the coating as the incorporated ions are reduced or oxidized. The preferential ejection of the more highly charged cation from fuUy loaded coatings (X, 0.5) during oxidation of O~(bpy),~+ to O~(bpy),~+ may reflect the dominating role of the electric field in the partitioning of counterions between the coatings and the solution.
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ACKNOWLEDGMENT This work was supported by the National Science Foundation and the U.S.Army Research Office. This paper is contribution no. 8510 from the Division of Chemistry and Chemical Engineering. W s t W NO.C, 7440-44-0;Os(bpy),2+,23648-06-8; OS@PY)~~+, 30032-51-0; NaOAc, 127-09-3; Ndion, 93615-63-5.
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RECEIVED for review September 19,1991. Accepted November 26, 1991.
Selective Ionophore-Based Optical Sensors for Ammonia Measurement in Air Steven J. West,?Satoshi Ozawa) Kurt Seiler, Susie S. S. Tan, and Wilhelm Simon* Department of Organic Chemistry, Swiss Federal Institute of Technology (ETH), Uniuersitdtstrasse 16, CH-8092 Zurich, Switzerland
Optical sewn (optodes) based on the incorporation of a m monlum Ionselective ionophores and hydrogen ion-selective chromoionophores in plasticized poiy(vinyi chloride) (PVC) membranes are applied to the measurement of ammonla In alr. Dynamic response characteristics and selectivities for ammonla with respect to other normally occurring gases under varying relative humldity are studied for several membrane formulations. No significant interference occurs from relevant ievds of SO2, NO2, or C02, but a trade-off between selectivity over other amines versus insensitivity to changes in rdatlve humidity is found. An optode formulated with the ionophore valinomycin, which forms a comparatively strong complex with ammonium ion, prefers ammonia over the aikylamines tested but ls affected significantly by humidity changes. An optode based on the bnophore EM 157, which fomrcl a weaker ammOnklm complex shows no hunldlty etlect but responds approxlmateiy equally to low levels of ethylamine, methylamine, and ammonia. I n the experimental configvatkn dascdbd, tlw latter optode has a range of 0.002 to 100 ppm, and f,, response times varying from 230 s at 0.05 ppm, to 15 s at 100 ppm,. A proposed optimization of the optical geometry promises to yield sub-ppb, detection limits and faster response times in future studies. There Is no deterioration In response after 4 months in laboratory air.
INTRODUCTION The measurement of ammonia in air is important over a wide range of concentrations. Ammonia in the atmosphere arises chiefly from natural s0urces.l Its concentration at ground level averages 0.002-0.010 ppm? and decreases monotonicallywith altitude? It is the only significant alkaline gas in the atmosphere, where its average residence time is only 7-14 days due to aerosol formation and neutralization by the more abundant acidic species.'l2 These reactions make the measurement of ammonia important in studies of smog and acid rain formation. Higher ammonia levels are of analytical interest in indoor environments where industrial operations such as refrigeration or fertilizer manufacture are carried out. The short-term exposure limit (STEL) for occupational exposure to ammonia is a time-weighted average (TWA) of 35 On leave from Orion Research Incor orated, Boston, MA 02129.
* On leave from Central Research Lagoratmy, Hitachi Ltd., KO-
kubunji, Tokyo 185, Japan.
ppm, over a 15-min period: and yet the lower l i i i t of human perception is 53 ~ p m , . ~ Numerous chemical sensors for the detection of ammonia in the gas phase or dissolved in aqueous solution have been described in recent literature. Primary sensing elements include solid-state,polymeric, and aqueous compositions. They operate by either surface or bulk-phase recognition processes and utilize optical or electrical signal transduction. A general problem with these sensors is that they are insufficiently selective for many applications; they respond to changes in relative humidity or to other relevant gases. Solid-state ammonia sensors based on changes in electrical properties of both metalized and nonmetalized metal oxidesemiconductor devices have been de~cribed."'~ They must operate at temperatures well above ambient (140-500 "C)or else suffer long recovery times, interference from changes in humidity, and poor limits of detection. Some of these sensors have been shown to respond to ammonia at ambient temperatures, but limits of detection are high (10 p ~ % ) ,and the effect of relative humidity is substantial.'*Js Sensors based on the effect of ammonia on the properties of organic thin films have been studied. The photoconductivities of various metal-modified phthalocyanines change in the presence of ammonia, but only at high levels in air (lo00 ppmv),16and there is a strong dependence on relative humidity.17 An investigation of thin films of N-docosyl pyridinium, TCNQ,18has shown that changes in its electrical and optical properties as a function of ammonia concentration are not selective or reversible. Polypyrrole, a conductive polymer, shows a decrease in conductivity when exposed to ammonia, but this effect is observable only at 100 ppm, and above, and response to nitrogen dioxide also occurs.1g Many sensors for ammonia are based on the reversible equilibration of ammonia with a thin film of aqueous solution, usually separated from the sample medium by a gas-permeable membrane. The change in an electrochemical or optical property of the solution is measured and related to the ammonia concentration. Potentiometric detection can be used to measure a pH change20or to directly measure the ammonium ionz1as in the well-known ammonia gas-sensing electrodes. Various amperometric detection systems have also been employed.22-24Incorporation of a pH-indicating chromoionophore or fluoroionophore into the aqueous film allows optical detection (optodes or optrodes), and signal transduction is usually accomplished with fiber optics.25 All of the aqueous schemes, however, can be applied only to the measurement of ammonia in equilibrium with an aqueous sample
0003-2700/92/0364-0533$03.00/00 1992 American Chemical Society
